This invention relates in general to polyimide resins and, more specifically, to compositions and methods for making low density resilient, flame resistant modified polyimide and polyimide-amide foams.
Prior U.S. Pat. Nos. 4,161,477, 4,183,838 and 4,183,839 disclosed and claimed certain polyimide compositions which are flame resistant and useful as coatings and adhesives.
The coating and adhesive compositions described in the above-mentioned prior patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxime. The ratio of oxime to dianhydride is preferably in the 2.3:1 to 2.7:1 range and the imidization reaction is preferably conducted at a temperature of 170.degree.-200.degree. C. for 20-60 minutes.
The polyimide forming material is then prepared by dissolving the bisimide in an inert solvent; then adding thereto a suitable diamine, producing a viscous fluid containing an intimate, unpolymerized mixture of N-substituted cyclic bisimide dicarboxlyic acid and diamine which is capable of being converted to a high molecular weight polymer by the application of heat.
The solution is coated onto a surface and polymerized by heating to a temperature in the 177.degree.-316.degree. C. range for 30 minutes to 5 hours. The following is exemplary of the exchange reaction which occurs: ##STR1## where n is a positive integer.
The resulting coating is tough, highly adherent to various surfaces, with very few pinholes or bubbles. It has excellent peel strength and is resistant to high temperatures, peeling and abrasion.
The prior coating material, however, was not suitable for use in applications requiring a cellular or foam material, since conventional agitation foaming and addition of known blowing agents add to process costs and complexity and are not entirely effective at the relatively high polymerization temperature required. Also, the density and volume weight of the prior foams tended to be undesirably high, especially in weight sensitive aircraft and space application.
Our prior U.S. Pat. No. 4,407,980 discloses methods of making superior modified polyimide foams. However, even these foams did not have the optimum low density characteristics necessary for many low weight structural applications.